Hi there.I'm not that much into biology, so sorry if my question seems stupid.

As an exercise I wanted to program a small application simulating a very simple biotope. Step one: some plants. They simply age, spread their seeds, and slowly cover all the area they can live on.Step two: herbivores. They eat the plants, maybe only young plants. Now here is the question: if those herbivores have no predators at all, what is the most likely outcome?

since the herbivores have no predators, their number increases and they eat up all the plants. Then those plants are gone and the herbivores die out as well.

as before, but some seeds of the plants survive and they recover in a biotope without those herbivores

there will be a balance concering the number of herbivores. Even without predators, their number will stabilize so they will be in a balance with the plants (what is the cause for this then?)

depends on the herbivore. Some will munch up everything and die out, some others have ony so few young anyway that they will never be enough to wipe out the plants.

combination of first three Their population will increase so much, that they eat all the plants, but some (both plants and herbivores) will survive and the population will start over. It won't reach exactly balance, but will oscilate around some number of plants and herbivores (plants will be peak earlier)

Well, as said it's not supposed to be a scientific model or give interesting results, but more a programming exercise combined with my amateur interest in nature sciences.

So the reason why some of the plants would survive, despite a huge number of hungry herbivores, is that the herbivores senses are limited and they wouldn't be able to locate all of those edible plants?

look for example on the grasshopers in Africa. They come in huge amounts, eat everything and go elsewhere. But does that mean, that it will be dead land for next five years? No, it will be affected and the population of plants will be reduced, but some of the plants survive, some may give rise to the seeds (although that depends, when does the herbivore peak).

Herbivores can't exist without predators. They have no way of regulating their own numbers so that they don't consume all the plants. In the scenario that they consume most of the plants and start dying off from starvation, giving the plants time to recover, there would be so much damage to their genomes that they could never become a successful and viable species. It is only with predators that this can happen.

Hi Jorgelobo. Sorry, I'm not good at exact terminology. But a population that goes through a cycle of near starvation every fifty years or so would be genetically unstable. A starving population tends to reduce in size. A well fed population increases. This regular unchanging yo yo effect would be very bad for any species.There is also of course the fact that without predators the beautiful herbivores we have today would never have evolved. They would all look more like tortoises without shells.

JorgeLobo wrote:Help me on this - how does starvation lead de facto to genetic instability and what do you mean be "genetic instability"?

Hi Jorgelobo. I studied ecology, not genetics, but I think there are rules for the survival of any species, and they are set up something like this:-There need to be Food chains, and the longer they are, the better. It's rather like all the bones in your arm giving a big range of movement. If you only had two joints there would be lots of things you couldn't do. A food chain gives a greater range of conditions for evolution, and leads to a greater diversity of forms. If the food chain is reduced evolution is slowed down for the species within it. Stagnation is something nature abhors. If a species can't evolve fast enough, it can't beat the disease organisms which keep ganging up on it. (Ok so crocodiles are successful, but they are a bit of a one off. The exception that proves the rule.)So to answer your question, What I mean by genetic instabiliy is a slowed genetic response to threats in the environment.

I think you overinterpret - evolution is not a near term event and dynamics of disease are much more complicated than you suggest - both for the host and parasite. In any case, evolution is not going to addres the short term starvation issue we were discussing.

Considering that there are Edible plants and non edible plants both, the alternation in animal population will cause as per food availability. Again that will depend on life span and size of animal. The animals with larger life span and large body size will definitely vanish. Animals with small body size will show the alternating population pattern.

Just because predators and parasites exist in world large and highly reproducing animal races including humans could survive longer in an ecosystem.

Senior Education Officer, BNHS, India. www.bnhs.org

Bitter Truth!Who says reason for world war IV will be Petrol? Reason lies in two words "Me and Mine".

I think Sachin makes a good point here. The smaller the animal the greater the number that will survive. Plus if there were no predators there would be no need for the animal to grow bigger, Perhaps the size wouldn't yo yo a much as I thought .On the problem of a change of plant type -eaten grass repaced by broadleaves, either the animal eats both or there are two species involved in the yo yo effect, a grazer and a browser. This wouldn't actually make a difference to the yo yo though. It would simply mean one species ate one area and the other the other. Both are going to run out of their food source as they increase in number, and they would be lucky to avoid extinction in the resulting desert.In fact, come to think of it, given a desert due to overgrazing coinciding wih a couple of years of poor rains it is very doubtful that any animal life would have developed on Earth without predation.